A device for measuring at least one characteristic parameter of a medium which at high pressure, and optionally also at high temperature, is bounded by the wall of a pressure housing or a pressure-resistant flow guide, wherein in the wall there is cut out crosswise a through-hole or through-going slot, wherein through the hole or slot there is passed a connection to at least one sensor which is responsive to said at least one parameter, and wherein the sensor and the electrical connection thereof are electrically insulated from the material of the wall and anchored to the wall by means of glass-ceramic material extending from the pressure side of the wall and wholly or partly through the hole or slot.
A device of this kind is known, inter alia, from U.S. Pat. No. 4587840, where the sensor consists of a pressure-sensitive resistance layer on the surface of the glass-ceramic material facing the medium that is under pressure.
Other devices are also previously known, wherein capacitive sensors are located in the wall, usually with a plastic material as insulation between the sensor electrode and the wall and also the said medium. However, plastic is usually unsuitable for this purpose when a medium is at high pressure, and especially also at a high temperature, and when the wall, for example, is made of a material other than plastic, such as steel.
It is also previously known to place capacitive sensors behind glass, e.g., sheets of glass, but this method is typically used in environments where there is approximately equal pressure on either side of the sheet of glass. It is also known that if such capacitive sensors are placed on the back of glass elements, e.g., sheets of glass, where there is substantial pressure difference on opposite sides of the sheet of glass, the thickness of the sheet of glass will be a function of the pressure difference in order to prevent glass rupture. However, it is also known that the sensitivity of the capacitive sensor in such cases diminishes notably with increasing wall thickness. The present invention allows this problem to be resolved in a simple and efficient manner.
According to the invention, it is therefore proposed that the device is characterised in that the sensor consists of at least one capacitive sensor electrode that is electrically insulated from the said medium by means of the glass-ceramic material, wherein the glass-ceramic material which both surrounds the sensor electrode(s) and the connections and hermetically seals the hole or slot is homogeneous.
According to another embodiment of the device, the part of the sensor electrode facing the said pressure medium consists of an electrically conductive layer, sheet or film and in shape is selected from the group consisting of: circular, annular, round, oval, rectangular or polygonal shape.
The device is particularly suitable for use when the wall in the pressure housing or the flow tube is made of steel or another pressure-proof, preferably electrically conductive material.
The thickness of the glass-ceramic material which distances the sensor electrode from the said pressure medium is preferably in the range of 0.1 xcexcm-10 mm, and the minimum thickness of the glass-ceramic material which distances the sensor electrode from the wall material is in the range of 0.1 xcexcm-10 mm.
In a preferred embodiment, the sensor electrode is connected to a sensor input on a capacitance-responsive signal processing circuit, and the wall material is connected to a reference input on the signal processing circuit.
In one variant of the device, the sensor element may be a part of a capacitive pressure sensor for measuring pressure in the said medium or cavitation effect caused by contaminants, such, e.g., sand, in the said medium, during movement thereof in the pressure housing or flow guide, wherein the sensor element consists of at least one sensor electrode, and wherein the glass-ceramic material surrounding at least one surface of the sensor electrode and the connections and hermetically sealing the hole or slot is homogeneous, and wherein the pressure sensor also includes a reference electrode which is in galvanic contact with the material of the wall, the sensor electrode and the reference electrode being spaced apart from one another and together enclosing a dielectric fluid, e.g., air, gas or a gel material or a dielectric, elastically yielding element.
The part of the said at least one sensor electrode which faces the said pressure medium consists of an electrically conductive layer, sheet, film or the like, and the reference electrode is galvanically connected to the edge of the hole or slot facing the pressure medium or the wall of the hole or slot.
The wall of the pressure housing or flow guide is preferably made of steel or another pressure-proof, electrically conductive material.
The reference electrode may, e.g., be placed in the hole or slot, at a distance from the pressure side opening thereof, the reference electrode on the side thereof facing the said pressure medium being coated in this solution with a layer of glass-ceramic material which covers the entire cross-section of the said hole or slot, the material preferably being flush with the pressure side of the pressure housing or flow guide. The reference electrode may be welded, screwed or snap-fastened to the pressure housing or flow tube.
The sensor electrode on the side thereof facing the said pressure medium is advantageously coated with a layer of glass-ceramic material which covers the entire cross-section of the said hole or slot, the material preferably being uniform with the rest of the homogeneous, glass-ceramic material.
The thickness of the layer of the glass-ceramic material which distances the reference electrode from the said pressure medium is in the range of 0.1 xcexcm-10 mm. Similarly, the thickness of the layer of the glass-ceramic material which distances the sensor electrode from the said dielectric fluid or element may be in the range of 0.1 xcexcm-10 mm.
The minimum thickness of the glass-ceramic material which distances the sensor electrode from the wall material is in the range of 0.1 xcexcm-10 mm.
The sensor electrode is connected to a sensor input on a capacitance-responsive signal processing circuit, and the reference electrode is via the wall material connected to a reference input on the signal processing circuit.